The invention relates to a method of compensating for the dark current of an electronic sensor having a plurality of pixels. In this case, radiation for producing a picture signal is directed from a radiation source onto a detector arrangement containing the sensor, and a plurality of recordings are taken with different recording time (integration time) through reading out the detector signal present in the pixels of the sensor. A method of this type can, in particular, be used to make dental panorama or cephalometric tomographs with an X-ray diagnosis instrument that contains a rotation unit with a beam source and, arranged diametrically thereto, a detector arrangement having an electronic sensor, preferably a CCD sensor.
EP-A-0 279 294 discloses the use, in dental X-ray diagnosis technology, of a beam-sensitive (CCD) sensor in panorama or cephalometric X-ray tomographs instead of a film with an amplifier sheet, and the electronic xe2x80x9creconstructionxe2x80x9d of the function of the moved film by a special type of operation of the sensor (TDI: time delay and integration), by correspondingly clocking forward the sensor""s charge packets produced by the illumination while new charges are being continuously added.
DE-A-19 525 678 describes a method and a device for adjusting picture-generating values in an X-ray that produces a panorama X-ray. A disadvantage in this case is that the spatial dependency of the dark current in relation to the individual pixels, which is due to the manufacturing process, is not taken into account. Although a time dependency is implicitly provided by adjusting the signal-to-noise ratio, the described arrangement nevertheless assumes an ideal sensor which works without defects in the pixels.
In design terms, X-ray sensors generally consist of a plurality of (CCD) elements arranged with minimal separation. The picture signal produced by X-radiation has a plurality of noise signals superimposed on it. The dominant impairment to the signal is made by the time- and temperature-dependent generation of charge carriers in the sensor, which is referred to in the specialist literature xe2x80x9cSolid State Imaging with Charge-Coupled Devicesxe2x80x9d, Albert J. P. Theuwissen, Kluwer Academic Publishers Dordrecht, Boston, London 1995, pages 92-95 (and pages 274/275 as regards TDI operation) as a dark current. The dark-current signal which is generated depends both on the sensor used and on the individual pixels of the sensor; the dark-current signal is therefore spatially dependent [y]. Furthermore, this signal is dependent on the integration time and therefore also on the variable turning speed of the rotation system, and thereby time-dependent [x]. In addition to this, the dark-current signal is also dependent ion temperature and the lifetime-cumulative X-ray dose.
Hitherto, corrections for the dark current have been made by picking up dark-current information at the start and end of a recording from the covered, that is to say unexposed, edge regions of the sensor. A correction value is calculated for each row from the data obtained in this way. The associated correction value, constant over the entire row, is then subtracted from each pixel of a row.
A disadvantage with this type of dark-current correction is that although fluctuations in the integration time, and therefore the time-dependent variation in the dark current are taken into account, no fluctuations in the individual pixels on a row of the sensor are taken into account. Significant picture artefacts can therefore occur depending on the CCD sensor used.
Another known method of correcting for the dark current is to subtract a full dark-current recording. To do this, in addition to the recording of the picture, a dark-current recording is made and is subtracted from the radiation recording. Although such a method gives good results, it cannot be used for the panorama and cephalometric recordings mentioned at the start since costs would be disproportionately increased through the dark-current picture and the concomitant storage costs additionally required. Furthermore, the recording procedure would be slowed by the required delay made necessary by the generation of position pulses.
WO-A-8 910 037 discloses a method and a device for compensating for the dark current and a base-value shift in the voltage of a CCD unit, in which a control process is used whose manipulated variable is derived from CCD cells arranged covered. This procedure is repeated continuously for each row of the linear CCD and the dark-current information of the covered CCD cells is used for correction of CCD cells arranged physically at other positions. In this case, the integration unit is operated with two time constants. To that end, the signals from CCD cells arranged covered are amplified and converted into digital signals. In a comparator unit, the digital values are compared with a predetermined value. If the value coming from the covered cells is too high, then the time constant of the integration unit is reduced. If the digitized value approaches the optimum, then the integration unit is operated with a large time constant, which results in faster read-out. It is furthermore proposed to assign the integration units a time constant which is so high that the amplified output signal remains approximately constant during the read-out of a row from the CCD unit. It is nevertheless possible to switch over the time constant of the integration unit from a first to a second position with smaller time constant, in order to implement the control loop, when the situation is far from the optimum dark-current compensation, as is the case for example when warming up the instrument or in the event of a significant variation in ambient temperatures.
A disadvantage here is also that only the CCD cells arranged covered are used for information about the dark-current profile and for the required dark-current compensation, rather than the CCD cells themselves actually used for recording the picture. Furthermore, the time taken to compare the digital values with a predetermined value at the start of each picture row leads to a delay in which no picture data are picked up.
The object of the present invention is to obtain an improved way of compensating for the dark current.
According to the invention, before the beginning or after the end of a recording with radiation, the sensor is read out without radiation using at least two different clock-out rates (integration times), and at least two dark-current signals are thereby picked up for each pixel. The dark-current signals that have been read out are then used to calculate a dark-current value of the individual pixels as a function of the clock-out rate and the calculated dark-current value available for each pixel is thereby calculated. This dark-current value is lastly used for picture correction by subsequently using the dark-current value associated with each pixel to make a correction to the detector signal with the dark-current signal superimposed on it in order to calculate the picture value.
Advantageously, a CCD sensor is driven using this method, in particular the operation of a two-dimensional pixel matrix in TDI mode being especially advantageous.
The sensor may in this case have a plurality of regions spatially separated from one another, between which no signal need necessarily be picked up. This non-radiation-sensitive region is advantageously minimal, approximately of the order of a pixel. It is thereby possible to produce a sensor composed of a plurality of regions. The response, deviating in particular in the edge region of each region in comparison with the central regions, can be corrected using the method according to the invention.
For calculating the dependency of the dark-current value on the clock-out rate, a computed relationship is produced with the aid of the at least two dark-current signals that are read out, and the dark-current value assigned to a pixel is calculated by means of a computed relationship corresponding to an inter- or extrapolation as a function of the actual recording time. The number of double-current signals read out for each pixel can be regarded in mathematical terms as a number of support points for ascertaining a computed relationship. The dark-current response of a pixel is in this case defined by an approximation, and the dark-current value of a pixel is expressed as a function of the integration time as a first-order equation, which is defined by two support points, or as a higher-order fitting function, if more than two support points are picked up.
In order to ascertain the integration times of the individual picture columns, the times of the clock-out pulses and therefore the clock-out frequency are picked up and kept. The integration time of a picture column in TDI operation is the sum of the respective last (n) ascertained clock-out times, (n) in this case defining the number of pixels in the TDI direction. The number of clock-out times depends on the width of the sensor and therefore on the number of pixels in a column. In a fictitious sensor with for example a 66 pixel width (in the TDI direction), 66 integration pulses are obtained. The integration times are calculated for each picture row from the preceding 66 integration rows and are stored.
The method is suitable, in particular, for taking a dental X-ray recording, since in this case a non-constant speed of motion of the sensor leads to different clock-out rates (integration times) of the sensor.
Advantageously, the method according to the invention is used to take dental panorama or cephalometric tomographs with an X-ray diagnosis instrument that contains a rotation unit with a beam source and, arranged diametrically with respect to it, a detector arrangement having at least one electronic sensor. During these recordings, the rotation unit is turned at a non-constant speed of motion around the patient""s head.
The detector signals obtained when the sensor is being read out are advantageously stored in a memory unit. In a subsequent step, correction to the detector signal with the dark-current signal superimposed on it is made in order to calculate the picture signal in a computer unit connected to the memory unit. By means of this, the compilation of the corrected recording after the generation of the uncorrected recording is decoupled from the actual recording process, so that the recording itself is not affected by the correction.
In principle, however, it is also possible to pick up the dark-current signals before the start of the recording and carry out real-time correction, so that picture values can be displayed actually during the recording.